Scatter is a major contributor to inaccuracies in the quantitation of PET images. The scatter fraction of current-generation PET scanners is typically 10-15% for brain-sized object is and can be as large as 30-50% with the inter-plane septa removed. Accordingly, a scatter correction must be performed in order to obtain accurate image quantitation. While a spatially varying deconvolution method is highly accurate for brain studies, the optimal method for two-dimensional (2-D) scans of the cardiac or torso which have spatially nonuniform attenuation media and for three-dimensional (3-D) studies remains less clear. One possible approach under study by several groups is the estimation of scatter through data acquisition in multiple energy windows. We have used a high-purity germanium detector with an excellent energy resolution to investigate the feasibility of energy-based scatter correction schemes for PET, SPECT, and planar nuclear medicine scans. The results of this study show a significant dependence of the scatter energy spectrum on the distributions of activity and scattering media. We aim to extend an energy-based scatter correction originally developed at NIH for single- photon emitting tracers to PET. This technique involves measurements of the energy spectrum under the photopeak. The practical issues (e.g., number and sizes of energy windows, spatial sampling of energy spectra) associated with this method will be addressed. The accuracy and variability of the correction will also be studied.